Studies to be described below are designed to provide information concerning the distinctive structural features of nucleosomes in active and inactive chromatin. Tetrahymena macro- and micronuclei have been chosen as a source of active and inactive particles by virtue of several unique advantages which they offer. Specificaly the amounts of histones and nonhistone proteins in subunits of active and inactive chromatin will be compared. In addition to careful quantitation of each component, appropriate analyses will be conducted to identify and compare the post-translational modifications which occur in the various proteins. Comparisons will also be made of the arrangement of nuclease sensitive sites within the two types of core particles and of the sites of interaction of histones and other proteins along the 145 base pairs of DNA. HMG proteins and ubiquitin will be isolated and their distribution between transcriptionally active and inactive chromatin will be measured. The cumulative data will permit construction of detailed models of the structure of active and inactive nucleosomes. Such models will provide a basis for future reconstitution experiments using defined DNA sequences: interaction of these particles with the transcriptional apparatus may then be examined with the aim of relating structure with function.